Wind turbine systems are often used to generate electricity from wind energy. One approach to implementing wind turbines utilizes constant rotational speed generators. In attempting to maximize the efficiency of a constant speed system, a difficulty arises due to the relationship of efficiency of the wind turbine to wind speed and turbine tip speed. More particularly, a given turbine blade typically operates at maximum efficiency when the blade tip speed is within a narrow ratio to wind speed. Turbine efficiency drops if the tip speed is either too low or too high relative to wind speed.
A known solution to managing tip speed is to use a variable RPM generator connected via fixed gearing to the turbine. This allows the generator to manage turbine speed by varying the load and thereby maintaining the turbine at an optimal speed. Some known variable RPM generators use expensive rare earth permanent magnets, increasing the cost of the generator. The energy produced by the variable RPM generator is of variable frequency and does not match the grid frequency (60 Hz), and therefore the energy must be adjusted through power electronics, adding significant cost to the system.
A known approach to reducing power capacity and expense of power electronics is to use a doubly fed induction generator, which has a fixed frequency stator and a variable frequency rotor fed by slip rings. Power electronics are only required to condition the energy generated from the rotor, which is typically 20-25% of the total generated power. A drawback to this approach is the use of slip rings, which may wear and need replacing. Another approach to reducing the power capacity and expense of power electronics is to use a brushless doubly fed induction generator which has both fixed frequency and variable frequency windings in the stator and induced currents in the rotor. A draw back to this approach is reduced overall efficiency of the generator because of the extra set of windings required.
With known wind generator systems, measures are put in place to manage high wind speed conditions in which the wind power provided exceeds the generation capability of the generator. In one example, during high wind conditions, the turbine is turned so that it does not face directly into the wind. This reduces turbine efficiency so that the turbine receives a fraction of the wind power and so the generator can still generate electricity. In another example, the wind turbine is locked so that it cannot rotate at high wind speeds to prevent damage from either overpowering the generator or from excessive turbine speed.
In a known turbine generator system, turbine blades connected to a rotor hub are driven by the wind and drive a low-speed shaft. The low-speed shaft drives a fixed ratio gearbox which then drives the high-speed shaft, which then drives the generator. A yaw drive and yaw motor work together to turn the turbine into the wind for maximum energy capture, and also to turn the turbine out of the wind if wind speeds get too high and the wind power transferred to the turbine exceeds the generator capacity. A disk brake provides and emergency braking and mechanical locking function. A cooling system prevents the generator from overheating, and an anemometer provides wind speed sensing for a controller.